The "Upload: Failed to write file to disk" error in WordPress indicates that the server is unable to save the uploaded file to the designated directory. This issue can be caused by various factors, but there are several steps you can take to troubleshoot and fix it:

1. Check File Permissions:

   • Ensure that the directory where you're trying to upload files has the correct permissions. It should typically be set to 755 for directories and 644 for files. You can use an FTP client to check and modify permissions.

2. Increase PHP Upload Limits:

   • It's possible that your PHP settings have a limit on the size of files that can be uploaded. You can try increasing the upload_max_filesize and post_max_size values in your php.ini file or by adding the following lines to your .htaccess file:
    

2. php_value upload_max_filesize 64M php_value post_max_size 64M php_value max_execution_time 300 php_value max_input_time 300

3. Check Disk Space:

   • Ensure that there is enough disk space available on your server to accommodate the uploaded file. If the disk space is full, you'll encounter this error.

4. Check the Uploads Directory Path:

   • In your WordPress dashboard, go to Settings > Media and make sure the "Store uploads in this folder" field is correctly set. If it's blank, try adding wp-content/uploads.

5. Check Server Quotas:

   • Confirm with your hosting provider that you haven't exceeded any storage quotas or file limits.

6. Temporarily Disable Security Plugins:

   • Security plugins can sometimes interfere with file uploads. Temporarily deactivate any security plugins you have installed and try uploading the file again.

7. Check for Plugin/Theme Conflicts:

   • Deactivate all plugins and switch to a default WordPress theme (like Twenty Twenty-One). Then, try uploading the file. If it works, reactivate each plugin/theme one by one to identify which one might be causing the conflict.

8. Check Temporary Folder Location:

   • WordPress uses a temporary folder for handling uploads. Ensure that this folder is correctly defined in your server's PHP configuration. You might need to contact your hosting provider to check this.

9. Check Error Logs:

   • Review your server's error logs for any specific messages related to the file upload issue. This can provide insight into the underlying problem.

10. Check Uploads Directory Ownership:

    • Ensure that the ownership of the uploads directory is correct. It should be owned by the same user that PHP is running under (usually the web server user like Apache or Nginx).

11. Contact Hosting Provider:

    • If none of the above solutions work, it's possible that there's a server-level issue. Contact your hosting provider for assistance.

Remember to always back up your website files and database before making any significant changes. This way, you can easily restore your site if something goes wrong during troubleshooting.